It's hard to figure out what's going since you said it comes and goes sort of. If you had and problem with the pump or hydraulic system failure you would have no assist causing very hard steering at slow speeds. Also have also seen you will only get assist in one direction. Sounds more mechanical to me check your pitman arm. Is it installed properly and all the way possible loose or striped splines on the output shaft. Just my 2 cents. Def something wrong. Wish I could check it out and see.